Electric hair cutting appliances are generally known and include trimmers, clippers and shavers whether powered by main supplied electricity or batteries. Such devices are generally used to trim body hair, in particular facial and head hair to allow a person to have a well-groomed appearance. These devices can, of course, also be used to trim pet hair or any other type of hair.
Conventional hair cutting devices comprise a main body forming an elongate housing having a front or cutting end and an opposite handle end. A cutting blade assembly is disposed at the cutting end. The cutting blade assembly usually comprises a stationary cutting blade and a movable cutting blade. The movable cutting blade moves in a reciprocal, translatory manner relative to the stationary cutting blade. The cutting blade assembly itself extends from the cutting end and is usually fixed in a single position relative to the main body of the hair clipper, such that the orientation of the cutting blade assembly is determined by a user orientating the main body of the device.
In common cutting blade units the cutting force driving the movable cutting blade is usually transmitted through a motor-driven eccentric. This eccentric is driven by an electric motor in a rotary manner. The rotary movement of the eccentric is then translated via a so-called driving bridge, which is connected to the movable cutting blade, into the resulting reciprocal, translatory movement of the movable cutting blade.
A common problem that occurs in such hair clipping devices is the so-called pulling effect. The pulling effect is an unwanted lifting of the movable cutting blade from the stationary cutting blade, which may especially occur during heavy load hair cutting. A reason for this pulling effect is the occurrence of a torque or twisting action on the movable cutting blade that may cause a tilt of the movable cutting blade. The evenness of the stationary and the movable cutting blade has a strong influence on the redoubtable pulling effect. It is therefore desired that the top surfaces of the cutting blades are as even as possible.
A lot of prior art hair clipping devices try to overcome this pulling effect by applying a very strong spring, which presses the two cutting blades against each other. The force applied by the spring shall impede a lifting or tilting of the movable cutting blade. The spring force is also used to compensate for manufacturing-related warpages within the cutting blades.
However, if the pressure between the stationary cutting blade and the movable cutting blade is increased, the friction between the two cutting blades will be increased as well. This increased friction often makes oiling necessary. Besides that it increases the abrasion of the two cutting blades. The increased friction also requires the appliance of an enlarged electric motor. Such an enlarged electric motor is on one hand expensive and on the other hand also voluminous. It increases the overall size of the hair clipping device as well as it increases the production costs. Apart from that, the power consumption of such enlarged electric motors is also higher than for hair clipping devices using smaller electric motors. This is especially disadvantageous for battery-driven hair clipping devices which in turn have shorter operating times.
Another approach for minimizing the risk for the pulling effect and improving the hair cutting performance is to provide cutting blades with sharper cutting edges. The cutting blades are usually provided with a plurality of teeth that act as a kind of scissor for cutting the hairs. The teeth geometry therefore plays an important role. Many prior art devices focus on an improvement of the teeth geometry of the movable cutting blade. However, also the teeth geometry of the stationary cutting blade, which is also denoted as guard, is of utmost importance.
Injection die casting processes allow to fabricate any desired teeth geometry with a synthetic material. Injection die casting is, however, a very cost-intensive manufacturing process.
Most of the prior art trimmer guard elements are made of metal, both for performance reasons and consumer appeal considerations. It is evident that metal guards have a longer lifetime compared to guards made from synthetic materials. Also their mechanical stiffness is higher. Nevertheless, these metal guards are more difficult to manufacture. Especially when thick metal guards having a thickness of more than one millimeter are used, creating precise and sharp teeth geometries becomes fairly difficult.
The state of the art manufacturing process for creating the teeth geometry of such metal guards is usually based on milling or grinding. In case of grinding, this is done by means of a regular grinding wheel with which the teeth are grinded tooth by tooth. This is, however, a very labor-intensive process. It has also been shown that the freedom of creating any desired teeth geometry is quite limited when using this grinding process.
FIGS. 8 and 9 show two examples of prior art stationary cutting blades (guards) with grinded teeth. These examples show that grinding the teeth of the stationary cutting blade limits the freedom in creating so-called scissor angles α in combination with sharp wedge angles γ. These angles are schematically illustrated in the figures, either in a top view (FIGS. 8a and 9a) or in a sectional view (FIGS. 8b and 9b). The scissor angle α is the angle with which the cutting edge of a tooth is inclined with respect to a vertical plane that is parallel to the longitudinal axis of the cutting tooth (see FIGS. 8a and 9a). The wedge angle γ is the angle between a lateral face and the top face of a tooth (illustrated in the cross sections A-A and B-B in FIGS. 8b and 9b). The scissor angle α is mainly important for the ability of the teeth to limit the amount of simultaneous cutting of hair in order to prevent an overload under heavy load conditions. Compared to completely straight teeth with a scissor angle α of 0°, slightly inclined teeth with a scissor angle α>0° show a better cutting performance. The wedge angle γ also plays a decisive role for the cutting performance to be achieved. A relatively small wedge angle γ leads to a very sharp cutting edge having an increased cutting performance with less required force. However a too small wedge angle γ (too sharp cutting edge) might lead to a mechanically instable tooth which is too sensitive for breaking.
The examples given in FIGS. 8 and 9 also show that the thickness of the guard material also limits the freedom of shape, meaning that the thicker the guard becomes, the more difficult it is to create a desired teeth geometry.
What can be seen from FIGS. 8 and 9 is the automatic dependency between these two angles α and γ which results from the grinding process that is usually used to manufacture the teeth. In the example shown in FIG. 8 the scissor angle α is fairly small or almost zero. This however results in a very large wedge angle γ near 90°, which leads to a quite unsharp cutting edge. However, by trying to sharpen the cutting edge, i.e. decreasing the wedge angle γ to about 45°, as this has been done in the example shown in FIG. 9, it is unavoidable that this also results in a relatively large scissor angle α of about 30°.
Creating a smaller scissor angle α while still keeping the wedge angle γ at a value of around 45° is not possible when manufacturing the teeth by means of a grinding tool. This results from the fact that a grinding tool usually follows a fixed geometrical logic with limited freedom. This means that when creating a small scissor angle α, a sharp wedge angle γ cannot be created with the grinding tool. Instead, when creating a sharp wedge angle γ, for example by a diamond dressed grinding wheel, a small scissor angle α cannot be manufactured.
The teeth geometry in grinded metal guards is therefore always a suboptimal tradeoff. This is especially the case for full metal guards with a thickness of more than one millimeter. These full metal guards however show a very good heat dissipation behavior due to their thick material and are therefore desirable.